Conventionally known stereoscopic image-capturing optical systems capture two parallax images of a single subject by dividing a single image-capturing surface into two (for example, see PTLs 1 and 2). In PTLs 1 and 2, two parallax images arranged in a direction perpendicular to the direction in which parallax occurs are captured. Thus, a stereoscopic image can be captured without sacrificing the resolution in the direction in which parallax occurs, which is important in stereoscopic image-capturing.